Custom bitarray [] and assignment operators

Question

I'm putting together a bitarray class for a GA thing I'm working on. I'm wondering if there's a better way to do the make my [] operator do assignments than what I came up with. Right now, I have the non-const version of the operator returning a secret 'bitsetter' class by value, which seems a bit excessive. I certainly can't return a bit by reference, but I'm wondering if there's a better (ie., more concise, efficient) way. Thanks in advance. Pardon my throw 0. Totally a placeholder ;)

class bitsetter
{
public:
    short ind;
    unsigned char *l;

    bitsetter & operator=(int val)
    {
        int vs = 1<<ind;
        if( val==0 ) {
            vs = ~vs;
            *l = *l & vs;
        }
        else
        {
            *l = *l | vs;
        }
        return *this;
    }


    int value() const
    {
        return ((*l)>>ind)&1;
    }

    friend std::ostream & operator << ( std::ostream & out, bitsetter const & b )
    {
        out << b.value();
        return out;
    }

    operator int () { return value(); }
};

class bitarray
{
public:
    unsigned char *bits;
    int size;

    bitarray(size_t size)
    {
        this->size = size;
        int bsize = (size%8==0)?((size+8)>>3):(size>>3);
        bits = new unsigned char[bsize];
        for(int i=0; i<size>>3; ++i)
            bits[i] = (unsigned char)0;        
    }

    ~bitarray()
    {
        delete [] bits;
    }

    int operator[](int ind) const
    {
        if( ind >= 0 && ind < size )
            return (bits[ind/8] >> (ind%8))&1;
        else
            return 0;
    }

    bitsetter operator[](int ind)
    {
        if( ind >= 0 && ind < size )
        {
            bitsetter b;
            b.l = &bits[ind/8];
            b.ind = ind%8;
            return b;
        }
        else
            throw 0;
    }
};

Seems fine to me. From what I recall, this is the approach recommended in Stroustrup. — Lily Ballard
– Lily Ballard, Commented Aug 24, 2012 at 20:33

GManNickG · Accepted Answer · 2012-08-24 21:01:46Z

2

This is the standard approach, it's called a proxy. Note that it's usually defined within the class itself:

class bitfield
{
public:
    class bit
    { };
};

Additionally, it's kept a little more "safe":

class bitfield
{
public:
    class bit
    {
    public:
        // your public stuff
    private:
        bit(short ind, unsigned char* l) :
        ind(ind), l(l)
        {}

        short ind;
        unsigned char* l;

        friend class bitfield;
    };

    bit operator[](int ind)
    {
        if (ind >= 0 && ind < size)
        {
            return bit(&bits[ind/8], ind % 8);
        }
        else
            throw std::out_of_range();
    }
};

So that people only get to see the public interface of bit, and cannot conjure up their own.

answered Aug 24, 2012 at 21:01

GManNickG

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